As a method for recycling a spent refractory, Patent Document 1 proposes a method wherein refractories used in an iron-making factory are subjected to e.g. sorting depending upon the respective sites of their use, then crushed and re-used as they are, as refractories. However, by such a proposal, the obtained raw material contained a large amount of impurities and could not be re-used as a raw material for electrofused refractories, which requires a level of the impurity content to be at least one digit lower than that of common bonded refractories, such that the required level of the impurity content is, for example, not higher than 0.1 mass % of Fe2O3 or not higher than 0.2 mass % of TiO2.
Further, by this proposal, the obtained raw material is simply crushed, and there has been a problem such that due to the crushing, the recycled raw material is likely to be re-contaminated, and such a recycled raw material is useful only as a low grade refractory. Among spent refractories, particularly, a spent electrofused refractory contains a large amount of a useful mineral source such as zirconia, and if it can be recycled again as an electrofused refractory, such will be an effective use of resource.
Patent Documents 2 and 3 propose methods wherein carbon is added to a spent refractory other than the electrofused refractory, followed by melt reduction treatment. Patent Document 2 proposes a method wherein a waste material of a chromium-containing refractory used in a steel-making process or a cement-manufacturing process, is subjected to electromelting treatment to obtain a solidified product having a spinel composition. However, with respect to a method for treatment of an electrofused refractory to be used for e.g. a glass melting furnace, no specific proposal is made. Further, the melt treated product is in the form of an ingot, and a post-process step of e.g. crushing is separately required to prepare refractory particles from the ingot, thus leading to not only an increase of the production cost but also a problem such that impurities are likely to be included in such a step of e.g. crushing thereby to lower the ZrO2 component and/or the Al2O3 component as useful refractory components.
Like Patent Document 2, Patent Document 3 proposes a method wherein carbon is added to a spent refractory generated in an iron-making factory, followed by melt reduction treatment. However, with respect to a treatment method for an electrofused refractory used for e.g. a glass melting furnace, no specific proposal is made, and further, since the melt-treated product is in the form of an ingot, a step of e.g. crushing is required to prepare refractory particles from the ingot, thus leading to a problem such that impurities are likely to be included during such a step. That is, by the methods proposed in Patent Documents 2 and 3, it was not possible to use a spent refractory as a raw material for an electrofused refractory, for which a high purity is required.
Further, as an important index required for an electrofused refractory, an oxidized degree of a constituting mineral may be mentioned. That is, if an electrofused refractory is constituted by a mineral in a state where oxygen of an oxide is partially missing, such a constituting mineral is likely to react with glass to generate bubbles thereby to contaminate the glass. In the methods proposed in Patent Documents 2 and 3, the melt-treated product is recovered in the form of an ingot, whereby carbon will remain at a high level in the recycled raw material.
The remaining carbon particles will work also as a reducing agent, and the recycled material cannot be used as it is as a raw material for an electrofused refractory, and it has been required to carry out heat treatment in the atmosphere, separately, in order to eliminate an oxygen deficiency.
Patent Document 4 discloses that wastes such as municipal wastes, industrial wastes or sewage sludges (they are usually vitreous with a large content of SiO2, CaO and Na2O) are melted for the purpose of volume reduction or detoxifying, but discloses or suggests nothing about adding carbon to a spent refractory which is not vitreous or increasing the purity of a spent refractory by adding carbon, followed by melting. Further, in Patent Document 4, the main invention is an invention relating to a lining refractory for a melting furnace.
Further, as a method for directly forming particles from a melt, Patent Document 5 proposes a method of blowing high speed air to a melt for air granulation. Patent Document 5 discloses that a raw material to form mullite, is melted to obtain mullite, but discloses or suggests nothing about a method of adding carbon to a waste, followed by melting to increase the purity of useful components.
In any case, there has been no proposal for an industrial method for treating a spent refractory to remarkably reduce components other than useful components such as ZrO2, Al2O3, etc. thereby to highly purify it and regenerate it to such a level as useful as a raw material for an electrofused refractory thereby to obtain high purity refractory particles.    Patent Document 1: JP-A-2003-212667    Patent Document 2: JP-A-5-96265    Patent Document 3: JP-A-2002-263606    Patent Document 4: JP-A-7-53258    Patent Document 5: JP-A-2003-251434